(A) Field of the Invention
The present invention is related to an over-current protection device and conductive polymer composition thereof, more particularly, to a positive temperature coefficient property of over-current protection device and conductive polymer composition thereof.
(B) Description of Related Art
The electrical resistance of conductive composition with the so-called Positive Temperature Coefficient (PTC) property is sensitive to the variation of temperature. Consequently, it is popularly used as a current-sensing device in over-current protection devices to protect battery and circuitry devices. Since the conductive PTC composition keeps a very low value of resistance at normal temperature, it will allow the circuitry and battery to work normally. Reversely, if the circuitry and battery meet over-current or over-temperature, its resistance will abruptly raise to a high value (at least above 104 ohm), and meanwhile, the over-current is reversely cancelled in order to obtain the goal of protecting the battery or circuitry.
In general, the conductive PTC composition is comprised of one or more crystallized polymers and the conductive filler. This conductive filler is uniformly distributed over the polymer. This polymer is normally a polyolefin (e.g., the polyethylene) and this conductive filler is normally the carbon black, metallic grains or inoxidized ceramic powder, for instance, the titanium carbide or tungsten carbide.
The polyolefin may be modified to obtain a used Poly Vinylidene Fluoride (PVDF), whose chemical structure of monomer includes a carbon chain, two hydrogen atoms, which link to the carbon of this carbon chain, and fluorine atoms linking to this carbon. This monomer is polymerized to form the PVDF. Usually, the fluorine possesses water resists and endures temperature variation; therefore, the PVDF has the characteristic of environmental attack proof.
There are many means for manufacturing the PVDF, and their properties are also different with respect to different means. However, their usual melting points are around the range of 160° C. to 180° C.
In order to enhance the performance of the PVDF in advance, it is possible to blend the PVDF with another polymer. For example, the Tetrafluoroethylene (TFE), which is a fluorine-based polymer called full fluorination, is used to reduce the electrical resistance of the PVDF blend after trip recovery.
In practical application, the over-current protection device usually faces more severe environmental conditions. For example, for those electromechanical devices located beneath the engine hood of a car, their design must consider that the engine is constantly running and also the climate outside the car to make the device expose under humidity and high temperature for a long time. Consequently, the over-current protection device has to increase the capability of humid proof and temperature varied endurance in advance.